1. Field of the Invention
The present invent ion relates to a guided-wave circuit module and a wave-guiding optical component obtained by connecting optical fibers to the guided-wave circuit module which are used in the field of the optical communication and more specifically to a guided-wave circuit module and wave-guiding optical component designed such that a guided-wave circuit on a guided-wave circuit chip incorporated into a holder can exhibit the characteristic properties thereof.
2. Description of the Prior Art
Development of a variety of high-quality guided-wave circuits such as light-splitters, optical wavelength multi-demultiplexers and optical switches in addition to the conventional light sources, optical fibers and photo-detectors has been desired as optical communication technique has been advanced. Moreover, these guided-wave circuits are required to have a large scale and high functionality, in addition to reduction of insertion loss and reflection at the connected interface between a waveguide and an optical fiber, and improvement in the heat stability. For these reasons, a guided-wave circuit has a size extending over the entire surface of a substrate having a diameter of 3 to 5 inches, and must accurately control the phase of propagating light rays.
In order to satisfy these requirements, it is important not only to improve characteristic properties of a guided-wave circuit chip per se, but also to optimize the structure of a guided-wave circuit module which supports the chip and to which an optical fiber is connected and, hence, providing the structure of a wave-guiding optical component obtained by connecting the guided-wave circuit module to an optical fiber. More specifically, there has been a requirement for the development of a guided-wave circuit module and a wave-guiding optical component which satisfies the following four requirements, i.e. , (1) accurate alignment of a waveguide with respect to a fiber; (2) accurate polishing of the endfaces of waveguides and fibers; (3) fitting of a guided-wave circuit chip to a holder without applying any stress to the chip; and (4) high mechanical strength. In particular, in the case of the foregoing guided-wave circuit module comprising a large scale substrate, the substrate inevitably warps. Thus, it is very important to support the substrate with a holder so that any stress is not applied thereto irrespective of the presence of any warp.
Further, in case of guided-wave circuits having active functions such as guided-wave circuits which make use of refractive index change of the guided-wave circuits due to a temperature change, i.e. , the thermo-optics effect and hybrid guided-wave circuits in which semiconductor devices are mounted on the substrate thereof, the guided-wave circuit chip must have good heat radiation capacity in addition to the foregoing four requirements.
An example of the structure of a conventional guided-wave circuit module will be given below and problems associated therewith will also be discussed in detail below.
FIG. 1 shows an example of a typical conventional guided-wave circuit module (See E. J. Murphy et al. , "Permanent attachment of single-mode fiber arrays to waveguides", IEEE, J. Lightwave Tech., LT-3, PP. 795-799, 1985). This module has a structure in which presser plates 3 are positioned on both upper edges of a guided-wave circuit chip 1 to connect optical fibers 2 and optical fiber arrays 4 are connected on both endfaces thereof to fix the optical fibers.
This structure for connecting optical fibers has a simple structure and, therefore, there has been widely used. According to this structure, it is possible to precisely align optical fibers 2 with respect to guided-wave circuits (guided-wave circuit part) 5. Moreover, the structure can easily be incorporated into the guided-wave circuit chip without applying any stress to the chip by using a small-sized guided-wave circuit whose warp can be neglected.
However, this structure has a low mechanical strength since guided-wave circuit chip 1 is exposed. In particular, in case of guided-wave circuit which is formed on a silicon (Si) substrate, the guided-wave circuit chip would easily be damaged and, thus, the reliability of the guided-wave circuit module is impaired. On the other hand, a large scale guided-wave circuit chip inevitably warps in not only the longitudinal direction but also the lateral direction. If this structure is applied to such a large scale chip, the warp in the lateral direction must be eliminated in order to fit presser plates 3 to the chip. For this reason, a great stress is applied to guided-wave circuit part 5 after the packaging thereof and accordingly the characteristic properties of the resulting guided-wave circuit are impaired.
On the other hand, Japanese Patent Application Laying-open No. 73208/1987 discloses a guided-wave circuit module having a structure in which a guided-wave circuit chip is not exposed. This module has a structure in which a guided-wave circuit chip 1 is sandwiched between two sheets of planar mounting substrates 6 and 7 and fixed thereto with an adhesive 8 to provide a guided-wave circuit chip unit 9 having unexposed surfaces of the chip as shown in FIG. 2 and in which two optical fiber arrays 4 for fixing optical fibers 2 are connected to both ends of unit 9 as shown in FIG. 3. If a guided-wave circuit module is designed to have such a structure, guided-wave circuit chip 1 is not exposed. Thus, the possibility of breakage of the guided-wave circuit chip associated with the module as shown in FIG. 1 would be substantially reduced.
However, when producing this structure, adhesive 8 must be applied to the whole surface of guided-wave circuit chip 1 to fix the chip to planar mounting substrates 6 and 7, stress caused by shrinkage of the adhesive during hardening is applied to the guided-wave circuit and, as a result, the characteristic properties of the guided-wave circuit vary after the chip is packaged. In particular, when guided-wave circuit chip 1 has a warp of a finite radius of curvature, the warp of the chip must be removed to sandwich guided-wave circuit chip 1 between planar mounting substrates 6 and 7. This results in the application of a large stress to guided-wave circuit chip 1. Moreover, if there is a difference in the linear expansion coefficient between planar mounting substrates 6, 7 and guided-wave circuit chip 1, stress is applied to guided-wave circuit chip 1 upon changes in the environmental temperature and, accordingly, characteristic properties of the chip are sufficiently affected by the environmental temperature change.
Structures similar to that of the module shown in FIG. 2 are disclosed in Herman M. Presby, Christopher A Edwards, "Packaging of glass waveguide silicon devices", Optical Engineering, 31 (1) , pp. 141-143 (January, 1992) and U.S. Pat. No. 5,076,654 entitled "Packaging of Silicon Optical components", Herman M. Presby, filed on Oct. 29, 1990. The article and the patent disclose packaged structures in which a UV-curable adhesive is applied onto the entire surface of a guided-wave circuit chip and a quartz cover is adhered to the upper surface of the chip through the adhesive layer. This structure suffers from the same problem discussed above in connection with the module shown in FIG. 2.
FIG. 4 shows an example of a module structure which allows self-aligning fiber connection between optical fibers 2 and guided-wave circuit chip 1 (see Japanese Patent Application Laying-open No. 234806/1988). To produce this module, level differences 1a for fiber alignment are first formed at both sides of guided-wave circuit chip 1 and a guided-wave circuit 5 is formed on the convex portion 1b existing between these level differences 1a (the central portion). On the other hand, a concave portion 10b, having a size corresponding to that of convex portion 1b, is formed on the side of mounting substrate 10 and the foregoing convex portion 1b is fitted in concave portion 10b. In this module, optical fibers 2 are connected to guided-wave circuit 5 of guided-wave circuit chip 1 through the self-aligning fiber connection. According to this module structure, guided-wave circuit chip 1 is fixed to mounting substrate 10 through the contact between level difference 1a on which any guided-wave circuit is not formed and mounting substrate 10. Accordingly, the shrinkage stress due to the hardening of a fixing agent, such as an adhesive or a solder, is not directly applied onto guided-wave circuit 5.
This packaging structure certainly makes it possible to save time required for aligning optical fibers 2 and guided-wave circuit chip 1, but the accuracy of fiber alignment is determined by the processing accuracy of level difference 1a of chip 1, or that of convex portion 1b of chip 1 and that of concave portion 10b on mounting substrate 10. The accuracy of fiber alignment thus achieved is in the order of 3 to 5 .mu.m while taking into consideration the production yield and the fiber connection loss is as much as 0.5 to 1 dB. Further, when packaging a guided-wave circuit chip having a warp of a finite radius of curvature, the chip must be fixed to a frame after eliminating the warp thereof. This is because, if the chip is mounted on the frame while remaining the warp of the chip, correct alignment between the fibers and the guided-wave circuit is not ensured at the edges of the guided-wave circuit. In other words, shrinkage stress caused by the hardening of a fixing agent is not directly applied to the guided-wave circuit, but a large stress generated by eliminating the warp of the chip is applied to guided-wave circuit chip 1. This leads to a significant variation in characteristic properties of guided-wave circuit 5.
As has been described above, it is difficult to package conventional structures of guided-wave circuit modules without applying stress to the guided-wave circuit chip, in particular to a large scale chip whose warp cannot be neglected. Thus, conventional techniques cannot provide a guided-wave circuit module structure which can satisfy all of the following requirements: accurate alignment between fibers and waveguides; support of a chip without applying any stress; and achievement of high mechanical strength.
Moreover, conventional techniques suffer from a common problem of accurate polishing of endfaces. More specifically, glass, silicon and metals, which are principal materials for forming guided-wave circuit modules and optical fiber arrays, have Young's moduluses of the order of 10.sup.3 to 10.sup.6 kg/mm.sup.2 which is 1 to 5 orders of magnitude higher than those of adhesives, such as typical fixing agents, which are used for fixing a guided-wave circuit to a mounting substrate. The guided-wave circuit module is constructed from a plurality of materials having different Young's moduli. When polishing the endfaces of a guided-wave circuit module and an optical fiber array in which a conventional typical fixing agent (adhesive) is used, the layer of the fixing agent having a low Young's modulus is first selectively ground and removed. Accordingly, the guided-wave circuit chip having a high Young's modulus, the optical fibers and terminal holders are exposed. If the polishing operation is further continued, the endfaces of the guided-wave circuit and the optical fibers are often damaged caused by abrasive particles which stay in recesses remaining after removal of the fixing agent. As has been discussed above, the polishing of the endfaces of guided-wave circuits having conventional structures and optical fibers leads to breakage of the guided-wave circuit chip and formation of defects on the polished endfaces. Therefore, good connection of endfaces cannot be ensured and, correspondingly, the fiber connection loss increases and the return loss deteriorates.
The conventional techniques also suffer from another common problem, that is, they do not provide any way for radiating heat from the guided-wave circuit chip. Therefore, it is difficult to apply the conventional technique to components which makes use of therm-optics effect and hybrid guided-wave circuits.